In this article I discuss the legality of Israel's interception of the Mavi Marmara on May 31 2010. Although Israel's stopping, boarding and inspection of the Mavi whilst on the high seas would undoubtedly constitute a violation of the law of the sea during peace time, I examine whether this violation can be justified on the basis of international humanitarian law. Specifically, Israel asserts that it was enforcing a naval blockade. I examine the legality of this blockade. I suggest that the blockade was unlawful on the basis that customary international humanitarian law permits the use of naval blockades only in times of an international armed conflict. I argue that on May 31 2010 Israel was not engaged in an international armed conflict with Hamas. Moreover, I submit that customary international law prohibits the use of blockades where they are intended to deny the civilian population objects essential for its survival or where the damage to the civilian population is excessive in relation to the anticipated military advantage. Israel argues that the intention of the blockade was to prevent war material from being delivered to Hamas fighters. This notwithstanding, I argue that because this blockade was causing a severe humanitarian crisis in Gaza on May 31 2010, it was incompatible with customary international law and therefore unlawful. Furthermore, even if the deployment of the blockade could be considered lawful, I argue that the enforcement of the blockade was unlawful because Israel's use of force to capture the vessel went beyond what was necessary in the circumstances.